While the scaling down of the device feature size in a semiconductor integrated circuit (IC) continues in the pursuit of smaller, faster, and more complex circuit functions on a single integrated circuit, maintaining high drive current at scaled voltages and smaller metal-insulator-semiconductor field-effect transistor (MISFET) gate dimensions generally becomes more important. Device drive current is closely related to parameters such as gate dimension, gate capacitance, and carrier mobility. Among the various technology innovations made to maintain high MISFET drive current, high-k (dielectric constant) gate dielectrics and metal gate electrodes are commonly adopted to increase gate capacitance of MISFETs in advanced technology.
Silicon oxide (SiO2) is widely used as gate dielectric layer for MISFETs. However, device feature size scaling in advanced technology may lead to a very thin gate SiO2 layer and, thus, the gate leakage current may become unacceptably large. High-k gate dielectrics may be used to replace SiO2 (k=3.9) gate dielectric in a sense that high-k gate dielectrics provide a thicker gate dielectric layer and hence leak less, while being able to maintain a desired large gate capacitance and thus a large device drive current. Drive current performance also may be improved through the use of metal gates. Compared to a conventional polysilicon (poly) gate, the use of metal gates tends to increase device drive current by eliminating the poly depletion effect.